Numerous cellular radio telephone systems have been implemented and are in commercial use in the U.S., Europe, and Japan. Most of the commercially operational systems are analog systems in that the messages are transmitted in analog form using a modulated carrier.
Boosters or repeaters are available for use with commercially operating analog systems. One such booster is disclosed in Leslie etal., U.S. Pat. No. 4,941,200, assigned to the assignee of the present invention. Such boosters, when installed in areas of poor or limited coverage, can substantially improve performance and customer satisfaction.
Newly evolving systems represent the messages in a digital format. These so-called "digital cellular systems" are attractive to operators in that they will improve the quality of communications as far as the user is concerned. They can also substantially increase user capacity as compared to presently available user capacity.
EIA Interim Standard 54 outlines a "Dual-Mode Mobile Station--Base Station Compatibility Standard". Dual-mode systems are capable of operating in either an analog mode or a digital mode. In analog mode, the conversations are FM modulated onto a carrier in conventional fashion. A single conversation requires a 30 Khz channel.
In digital mode, the conversations are digitized according to IS-54, and transmitted digitally. By using time division multiplexing, three to six digital conversations can be multiplexed onto a single 30 Khz channel.
IS-54 specifies a control channel scheme which is upwardly compatible with the prior EIA 553 specification which outlines AMPS-type analog only compatibility. Analog only portable cellular units or mobiles can operate on a dual-mode system without modification.
When a movable unit accesses a dual-mode system, part of the service request message indicates the movable unit's ability to operate in digital mode. The cellular system can then assign the movable unit to a digital traffic channel and time slot within the channel. In the digital mode, the movable units' transmissions are relatively short bursts of data aligned such that 3-6 movable units may use a single channel.
Known repeater or booster structures, such as disclosed in the above-noted Leslie et al. patent, are intended to be used with commercially operational cellular analog systems. As such, they do not have the capability to properly retransmit or boost digital messages or conversations of the type envisioned by the above-noted EIA Interim Standard 54.
Thus, there continues to be a need for booster or repeater systems usable in radio telephone systems which are compatible with dual mode analog and digital, or completely digital cellular radio telephone systems. Preferably, such boosters would be usable with existing analog systems on an interim basis, if desired.